Interactions between the green peach aphid, Myzus persicae (Hemiptera: Aphididae), the entomopathogenic fungus Lecanicillium muscarium (Ascomycota: Cordycipitaceae), and a common parasitoid Aphidius colemani (Hymenoptera: Braconidae), were tested with Brussels sprout (Brassica oleracea var. gemmifera L. cv. Bedford Winter Harvest) as host plant, under laboratory and controlled field conditions. The effects of temperature, relative humidity and developmental stages of M. persicae on the efficacy of this fungus were measured under laboratory conditions. The most effective method of co-applying these two biological control agents to enhance their additive interactions against M. persicae was also assayed, in particular to determine if fungal infection of the aphid host had an effect on the number of aphids mummified by the parasitoid, and on emergence rate and parasitoid sex ratio. Although this fungus can be grown at a broad range of temperatures (15–30°C), the optimum temperature for control of M. persicae ranged between 20°C and 30°C. Lecanicillium muscarium had high efficacy as a microbial control agent against M. persicae between 55% and 90% RH. Furthermore, the fungus was most virulent against adults and older developmental stages, compared to young developmental stages of M. persicae. Results also showed that although fungal infection has no negative effect on the rate of nymph production per day, total fecundity of infected aphids declined overall as the length of the reproductive period of the aphids was reduced. In no-choice and choice experiments under laboratory conditions, mummification, adult emergence from mummified aphids and female sex ratio in emerging adults, were unaffected when the fungus was applied 6 or 7 days after the parasitoid was added, compared with treatment by A. colemani alone. However, all parameters were significantly lower when L. muscarium was applied first, compared with treatment by A. colemani alone. Co-application of A. colemani or exposure to naturally occurring parasitoids and L. muscarium can interact positively, leading to increase the control level to be higher than equivalent level produced by one agent alone but less than additive effects in a field cage experiments. Laboratory experiments showed that aphids feeding on rye were the most susceptible to infection by L. muscarium. The highest percentage of aphids mummified and emergent parasitoid females were found in aphids feeding on green pepper. Further research is required in large scale open field conditions to study the combined effects of the fungus and either naturally occurring or introduced parasitoids on aphid population growth. The implications of these results for the use of L. muscarium and A. colemani in combination as a biological control programme are discussed.